Purification of benzoic acid and its derivatives



Patented July 18, 1933 UNITED STATES l traan ENT ortie `PAT ALriroNs o. JAEGER, or, onAr'roN, rnNNsYLvANrAfAssrenon To fri-rn sELDEN l coMrANY, or rrrrsisnnsn, rnNNsLvANrA, A- coRronArroN or DELAWARE rURIFIoATIoN .or BENzoro Acrn Ani) rrs niiarvArlvEs L No Drawing.

benzoicacid and its derivatives kand more lli) particularly to their separation from the corresponding phthalic acids.

Benzoic acid and its deriva-tives, such the amino-, nitro?, chlorand -brombenzoic acids may be prepared by various processes from the corresponding phthalic anhydrides or phthalic acid and salts, for example' by heating an aqueous vsolution under pressure with or Without a catalyst and/or the presence of reducing gases, vapor phase catalytic reactionin the presence of steam With or Without reducing'gases, heatingfsalts in the solid or pasty phase, vapor phase reactions in the presence of electric discharges with or Withoutreducing gases, etc. The product obtained ordinarily contains .both benzoic acid and unchanged phthalic acid in varying amounts. The separation of these ltWo products presents considerable technical diiiiculty.

The standard methodr which Was used ink the past consists in preparing awater solution of thetwo acids and then removing the benz'oic acid by solvents such as ether, chloreform, and the like. As large amounts of solution have to be handled,this method, although giving a reasonably good separa- 1fion, is tooexpensive forpractical use.

In my prior Patent No. 1,685,634 dated September Q5,k 1928, I have described and claimed a'method of separating benZoic acid l from phthalic acid in aqueous solution by countercurrent lovv of solvent for benzoic acid `Which'does notmiX with Water and this process can becommerciallyused especially where the benzoic acid and phthalic acidsare obtained normally in the formbf a Water solution or suspension.v `Where, however, they are obtained inthe Yform of a mixture of solids, either dry `or only containing relatively small amounts or Water, it is necessary first to-dissolvethem up inwater, a tedious process necessitating largeA apparatus because of the low solubility of the acids.

The present inventioneliminatesthis disadvantage land is based on the discovery that7 contrary to the teachings 'ofthe past, Vit vis not' necessarvto dissolve the acids in Water in order to obtain a clean separation by .means of a solvent .for benzoic acid and, on the contrary, the benzoic acid can be Washed out from the corresponding phthalic acid by means of a. solvent such as chloroform; aroasl y which Application med April 1i, 1929. Lserifn'm. 352,611.

matic hydrocarbons, tor instancebenzol,l toluol, solvent naphtha, carbon tetrachloride; chlorinated hydrocarbons, such as'for eX# ample, trichlorethylene, tetrachlorethane; and lsome of .the chlorinated residues or sludges produced as a'by-productfrom lthe purification of `aromatic hydrocarbonsby selective halogenation and described in my prior Patent `Noy yl,?'41-,3()51 dated 'De- Y cember 31,1929 and co-pending application Serial No. 101,561 iiled April`12, 1926; ether; parafnhydrocarbons; esters; and other Well known organic solvents of similar character. `Even Whenthe solid mixture is substantially dry'or contains Ibut very little Water, a' satisfactorily clean separation may be'obtained'asthe phthalic acid, contrary to the belief in `the past, is not suiiiciently soluble either in the Vsolvent orfin the benzoic acid to be Washed out in'quantities sulicient to cause any trouble. Y

The Washing orleaching process lof the present invention produces a clean-cut sepa*-v ration in apparatus of small dimensions With'- out the necessity of treating -anjen'ormous quantity or" water as in `the .processes used in the past, and a great increase in 'speed is obtained as the process is `ideally suited for rapid filtration or Washing such asis possible ina centrifuge. A solution of benzoic'aci'd is obtained from which the solvent' canbe readily evaporated and when a suitable solvent such as aromatic hydrocarbonf'or the like is chosenan azeotropicinixture is'formed lwith* any small amounts of Water presentand a perfectly dry benzoicacid is obtained which for some purposes does not even need to be sublimed, an important technical economy.

in a most satisfactory form with la minimuln Y of diiculty.

`Further advantage of the present invention lies in the fact that it can be carried out with the material in a vsubstantially dry` state y greatly decreases the corrosion problem for an aqueous solution of phthalic acid is quite corrosive to some metals such as l iron and necessitates either the use of special apparatus or results in a high depreciation'.

The Washing may take place in any suitable manner, but I have found that it is desirable in most cases to use a centrifuge Which gives avery rapid separationf'vvith' apparatus ofv small dimensions. The mixture of phthalicacids and benzoic acids may be placed in a centrifuge, the centrifuge started and a solvent such as benzol permitted to flow in, or, especially Where the mixed product is obtained in the form of comparatively dry solid by catalytic or other processes, it is desirable to mix it with sufficient solvent to `form a paste or slurry'suiiiciently thin to be readily handled. In this manner the centrifuge 'opcrates more rapidily asv the solvent is intimately mixedivvith theproduct before the Washing commences.` Additional solvent is, ofcourse, run in to 'the center of the centrifuge until the filtrate is substantially free from benzoic acid.

A single solvent may be used or in some cases it may be desirable to use either a mixture of solvents or a succession of solvents. Thus, for example, Where it is desired to make a'4 paste of the crude product for ease in handling this maybe made `with a suitable non-inflammable solvent suoli as carbon tetrachloride, `the pasteV 'placed -in a closed centrifuge, centrifuged until substantially dry, and then further Washed with infiammable 'solvent such as benzol, ether, or the like, thus eliminating all fire hazard in transferring the paste to the centrifuge While permitting the use of highly inflammable solvents Where these are desirable. V

The Washing or filtrationmay be at ordinary room temperature or, if desired, may be at suitable elevated temperature, rbut care should be taken that the temperatureis not sufficiently high so that phthalic acid is dissolved. -This Will vary, of course, With the dierent solvents and in no case should the temperature exceed 1910 C. as above this temperature the phthalic acid is transformed' into phthalic anhydride, which is quite soluble in most of the organic solvents for benzoic acid.

Insteadof a centrifuge anA ordinary filter.

press or nutsch filter may be used but the process is less rapid and for mostpurposes it is desirable to use the morel efficient and more rapid-type of apparatus. Continuous filters, such as for example,those of thel Oliver type may also be used, and present some advan.

tages as the process is thena continuous one instead'of a batch process. On the other hand the use of filters of this type necessitates the thorough mixture of the crude material With the benzoic acid solvent and very slow operation in order-to assurecomplete removal of the benzoic acid. In most cases, therefore, a centrifuge although necessitating a batch .process-A will be found to present technical ltentcan be handled effectively Whichfpresents some diculty in a continuous filter. The

'invention is, therefore,'not limited to the use of any particular filter or filter type and the chemical engineer Will choose the type best suited for the particular installation and for the particular characteristicsof the phthalic and benzoic acid-mixturefwith which he is dealing. Y.

lVhat is claimed as new is:

1. A method of removing benzoic acids from a mixture of benzoic and phthalic acids in which at least part of the acids arepresent in the solid phase, Which comprises Washing the mixture with a solvent essentiallycomprising a hydrocarbon of the benzene series.

2. A method of obtaining dry benzoic acid from a mixture of benzoic and-phthalic acids in which at least part of the acidsv are present in the solid phase and which mixture contains moisture, which comprises Washing the mixture with a solvent essentially comprising a hydrocarbon of the kbenzene series and subjecting -the solution to distillation at a temperature which `drives oif the hydrocarbon and the azeotropic mixture of Water andV hydrocarbon formed, and continuing the distillation until substantially dry 'benzoic acid Vis obtained.' f

3. A method of separating benzoic and phthalic acids, .Whichcomprises forming a substantially drymixtureof the two masses into apaste with a solvent for the particular benzoic acidv and subjecting thepasteto leaching With' a hydrocarbon of the benzene series. v y y 4. A method of removing benzoic acids from a mixture of benzoic and phthalic acids inwhich at least part of the. acids are present in the solid phase, which comprises centrifugingl said mixture in the presence 'of a hydrocarbon of the benzene series.

VK5. A method of removing benzoic acids from a mixture of benzoicand phthalic acids in Which atleast part ofthe acids are present the mixture With solvent naphtha.

4ALPHONS O. JAEGER. 

